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"Workers at Fukushima Daiichi were working this week to remove rubble from atop the crippled Fukushima Daiichi Unit 3 reactor building with remote-controlled cranes, when they discovered the radiation dose from a piece of debris they removed from the 5th floor of the reactor building was highly radioactive and releasing a dose rate over 540 millisieverts per hour. The maximum allowable exposure for radiation workers in the U.S. is 50 millisieverts per year.

The debris, which is approximately 1 cubic meter in size, was transferred to a building on the south side of the Fukushima Daiichi complex for temporary storage, until it can be transferred to a solid waste storage site in the future."

No doubt the Japanese investigators have sampled the debris to find out what it consists of, so hopefully within a few weeks we should be given more information on this.

The UK's Office of Nuclear Regulation (ONR) keep telling us all that the explosion at Fukushima Daiichi Unit 3 was a simple hydrogen explosion. I have spoken to two experienced Nuclear Inspectors face to face on two different occasions. At the time of our conversations they both supported the consensus hydrogen explosion thesis.

However it is now very likely that as the on-site teams explore further into the building they will find even higher levels of radioactivity than just found on the 5th floor.

How can a hydrogen/air explosion account for such large lumps of highly radioactive debris?

This debris must contain significant amounts of material from either inside the reactor containment or from within the fuel pool; there are no other sources for material as radioactive as this.

The evidence is really starting to stack up against the consensus view of Nuclear Regulatory Professionals across the world have adopted; that this was just a simple hydrogen explosion.

The roof steels are all missing right above the unit 3 fuel pool, and given that the measured radioactivity levels, within the reactor dry wall containment, did not change significantly around the time of the explosion, it seems to me that a very credible explanation is that some form of fuel pool detonation occurred.

However the problem is we won't know for sure what happened until new robots are built that can get into to inspect the internal building damage in detail. The radioactivity levels are too high for humans and they may well prove to be too high for existing robot technologies as well.

In the meantime I would like to see the HSE withdraw HM Chief Inspector's Fukushima Final Report findings in regard to the Unit 3 reactor explosion, pending further data and a further evaluation of the newly available facts.

I don't want to see the ONR have to withdraw their report findings, after pressure is applied by campaigners and politicians, and in a way that completely undermines their future credibility.

On page 3 of this document are some typical source dose rates. As these sources are typically 1 or 2 mm in size, this dose rate from a 1m cube is not a very intense. It is certainly not part of the core or a fuel rod as you are suggesting.

You sound more certain about this than the
ONR. The radioactivity on and/or in this concrete block is certainly due to
radioactive fission products. (i.e. is definitely not originating from the
pulsed output from an industrial x-ray source)

It represents roughly a 1000 fold reduction in dose radioactivity
compared with an intact spent fuel rod bundle. (the 1000 fold reduction is
obviously linked to the dispersion process that occured along with the
explosion)

We will have to wait and see whether this is hype or not.

James Arathoon

(Now that the above posts have exceeded the width of the allowed space,
you have to copy and paste the text into a text editor to read it.)

As I said, there is insufficient information in this article to make a real judgment.

We know that a quantity of fission products would be vented from the reacor during the postulated hydrogen explosion. If this (unspecified) item was a filter or part of the vent ducting it could hold sufficient of the vented fission products to achieve these dose rates.

What sort of spent fuel pool explosion could cause this? You could have enough reactivity to boil the water and cause some form of steam bubble. As the pool is open topped it would be hard to build up enough pressure to cause a noticable explosion.

If the pool dried out it is possible that the cladding on some freshly removed fuel rods could catch fire. The fuel itself can't burn as it is already in oxide form as far as I understand. Again how do you get an explosion?

If some fuel assemblies became exposed and caught fire then the racking could have failed (especially if it was boronated aluminum alloy racking rather than boronated steel racking - we have not been told yet which it was).

One possibility is that a stream explosion arose out of a the zirconium fuel rod cladding fire, with the high temperature and molten fuel assemblies collapsing into the remaining, but still boiling water. The explosive power of a molten metal steam explosion is immense so this is one possible explanation.

The other option, which still can't be discounted, if the racking melted and the fuel assemblies collapsed, is some sort of nuclear criticality arising from the fuel assemblies collapsing together.

Most of the energy and momentum from the unit 3 explosion was directed upwards.

How a hydrogen explosion in air could do this with the 5th floor side wall structure being more flimsy than the roof is beyond me.

There may of course have been considerable upwards momentum if the reactor itself blew its top, however the sensors do not show pressure changes in the reactor or containment capable of doing this. So this theory can be discounted, unless of course the publicly available data is wrong, which is unlikely.

I believe the thick and substantial concrete fuel pool side walls acted as a sort of shot gun barrel directing most of the energy and momentum of the fuel pool explosion upwards.

I wrote to the ONR and DECC in August 2011, outining my thoughts on all this. I have now written to the ONR again. They have just told me that the Fukushima team will repond in due course.

We know that cooling was lost. We know that at high temperatures the zirconium fuel cladding catalyses the breakdown of water producing hydrogen. We know that the operators were obliged to vent the primary containment vessel due to high pressuure. Venting the PCV will fill the upper part of the building with hydrogen and a variety of radionuclides. Hydrogen is explosive. The hydrogen is ignited, the shock wave destroys the upper part of the building ajnjd drops rubble into the spent fuel pool.

You don't need complicated theories from anti nuclear activists like Gundersen who is trying to make out that a criticallity event occured and the govenment is hushing everthing up.

Whatever happened at Fukushima Unit 3, big lumps of material got thown up and out, as shown by the holes in the roofs of the nearby buildings.

The fact that the explosion did not arise in the fuel pool does not mean we have solved the problem, just that we have eliminated some potential root causes.

Before being too damning of Arnie Gundersen, look at what he and his wife Maggie have done in defending the public interest in terms of what has gone on at San Onofre with the Steam Generator replacement fiasco. Look at what John Large in this country is doing in this regard as well.

Significant parts of the established Nuclear Industry across the world have become their own worst enermy, and it is important that the rot of secrecy, lies and deception be completely dug out, so it doesn't infect the thought and action of future generations.

Even if Fukushima had exploded in a similar way to Chernobyl why would that be considered more significant than for example the Bangladesh Dhaka building collapse? Why is this recent Fukushima news significant? If we speculate on what could have been the worst possible failure event at Fukushima, then that still pales into insignificance compared to the deaths over the years caused by other forms of energy production or large scale industrial processes. There are many large scale events, fires, explosions, ship losses, dam bursts, air crashes, chemical leaks etc, that have been far worse in terms of loss of life and destruction than even Chernobyl.

Deep Water Horizon is a big factor in my thinking as well. Companies are not just liable for death and injury their inaction or action causes. Companies have a huge reponsibility in terms of maintaining and respecting the natural and economic environment as well.

The calculations that the Office of Nuclear Regulation (ONR) make on risk and consequence do not properly factor in the costs of economic and environmental damage. Since the Deep Water Horizon Disaster it has become clear that they are out of step with the rest of society on this and as such their methodologies should be reformed.

In my opinion the ONR's methodologies can't be properly reformed for the better without moving to intrinsically and passively safe fourth generation nuclear technologies.

Arnie Gundersen does not defend the public interest. He and the other well-known activists are responsible for a large number of stress induced health problems and deaths! By prophesying huge numbers of deaths and greatly exaggerating the hazards of low level radiation they vastly increase the stress levels in the population. Stress is a major cause of heart attacks and other health problems.

There is an increase in the heart attack rate around Fukushima as stated in this fine anti-nuclear article:

I would propose stress, increased by views such as these as, the major cause aided by the large amount of chemical pollution caused by the tsunami.

The greatest actual loss of life from Chernobyl was the 100,000+ unnecessary abortions performed due to this scaremongering.

"In many countries in western Europe, legal abortions increased for several months following the Chernobyl accident probably because of fear of the effects of the radioactive fall-out (see, e.g., Fig. 2.6); according to the IAEA, 100,000 to 200,000 excess abortions were performed throughout western Europe after the Chernobyl accident (Ketchum, 1987)."

Before you can reform the risk methodologies you need a sound basis for measuring the risk. There is currently no proven measure of risk for low dose radiation. The existing Linear No Threshold theory, LNT, is just that, a theory. It is not supported by experimental or epidemiological results.

Without the correct information it is not possible to plan things like evacuations and exclusion zones. If 10 times or 100 times the current dose limits cause no significant increase in risk, which is likely, the need for evacuations and the size of exclusion zones fall dramatically.

Originally posted by: jarathoonMaintaining public trust in the integrity engineering profession.

Deep Water Horizon is a big factor in my thinking as well. Companies are not just liable for death and injury their inaction or action causes. Companies have a huge reponsibility in terms of maintaining and respecting the natural and economic environment as well.

Well yes, that applies to many major industries, but I often find a complete lack of perspective and quite a lot of hysteria with anything nuclear related. If a nuclear spent fuel flask is transferred by road then protestors and TV cameras appear, and tests have to be done like crashing a train into a flask at full speed. There is never the same level of attention applied to the transport or storage of dangerous chemicals, why not? Even the transport of fuel through congested city areas is taken for granted, yet a tanker explosion in such a situation is devastating, has happened many times, yet rarely stays in the news for more than a day or two.

In the case of Fukushima, I find the events and engineering challenges quite fascinating, but am bewildered at suggestions (not necessarily by you) that this is any more dangerous than many other large scale industrial processes.

A naive and quick attempt to partially classify the pollution and associated health effects problem:

1. Various forms of illness, disease and death can result from chemical as well as radioactive chemical pollution in the environment. In particular radioactive pollution can be a chemical, as well as a physical, cause of illness, disease and death. (physical in the sense of alpha, beta or gamma radiation smashing into biological molecules and changing their structure).

2. Separating and quantifying the effects of chemical and radioactive chemical pollution is extremely complex. Chemicals enter our bodies in various different ways and we all end up our own unique cocktail of chemicals in our bodies, as well as a unique DNA make-up, that determines exactly how these chemicals are used, stored or excreted by the body.

3. When pollutants enter the environment they are gradually dispersed throughout it. There are a wide variety of dispersal mechanisms: physical, chemical and biological. These dispersal mechanisms have distinct physical, chemical and biological characteristics and all have their own associated time constants.

4. The scientific and statistical problem of how we relate exposure to various chemical pollutants in time and space to the observed disease and illness distributions is a complex, ill defined and ultimately unsolved problem.

5. Sometimes the dispersed pollutants are re-concentrated back up the food chain in surprising ways, with maximal concentrations found in animals at the top of the food chain.

6. Time relationship between exposure chemical pollutants and their eventual effect in causing disease can be quite complex. For example pollutants can be gradually and harmlessly stored away in our body's fatty deposits over time, but later released to harmful effect if we lose weight for some reason.

Given the complexity of all this, it is not surprising that there is lots of scientific debate, and lots of potential for academics and commentators on all sides of the debate to talk nonsense.

I think as engineers have to try and clarify the nature of the problem and then try to be as clear as possible about what is likely to happen if we take a particular course of action; for example by reusing steel from existing nuclear power stations to build new power stations. One cost is that some extra radioactivity will disperse into the environment as a result of this decision; one benefit is that we get to reuse steel which would otherwise go to waste and our new nuclear stations are slightly cheaper as a result.

I suspect that a significant part of the argument comes from the RDD (dirty bomb) side. If you can greatly reduce the disruption and fear effect from an RDD, no one will go to the additional risk and effort to make one.